The present invention relates to a power factor control system for AC induction motors and, more particularly, to a self-calibrating power factor controller for AC induction motors.
A power factor control system for AC induction motors is disclosed in U.S. Pat. No. 4,052,648 issued to Frank J. Nola on Oct. 4, 1977 and assigned to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration ("NASA"). The Nola patent is contained in a NASA Technical Support Package dated March, 1979 and entitled "Power Factor Controller, Brief No. MFS-23280". In addition to the Nola patent, the Technical Support package contains schematic diagrams of variations and improvements on the circuitry disclosed in the Nola patent.
As explained in the Nola patent and in the NASA Technical Support Package, the current in an AC induction motor may lag the voltage by a phase angle of 80.degree. when the motor is unloaded and by 30.degree. when the motor is loaded. This phase angle ".theta." is used to compute the power factor for the motor, which is defined as cos .theta.. Thus, when .theta. is small the power factor approaches 1. Conversely, where .theta. is large the power factor approaches zero. Fundamentally, a low power factor means that energy is being wasted. Given the tremendous numbers of AC induction motors in use today, improving the power factor could result in very substantial energy savings. Estimates of potential energy savings are set forth at pages 3 and 10 of the NASA Technical Support Package.
The operation of the Nola power factor controller is described in the NASA Technical Support Package at pages 11 and 15 using the functional block diagram appearing at page 13. The line voltage is sensed and signals corresponding to the line voltage and its complement are produced. The motor current is also sensed and signals corresponding to the motor current and its complement are also produced. An "EXCLUSIVE OR" logic operation is then performed on these voltage and current signals, the result of which is one input to a summing amplifier and low pass filter. The other input is a DC signal, derived from a potentiometer, which corresponds to a commanded phase angle and, therefore, a commanded power factor. The result of this filtering and summing operation is a DC system error voltage which is then compared with a ram voltage synchronized with the zero crossings of the line voltage. The intersection of the ramp voltage with the DC error voltage is detected by the comparator and used to trigger the triac. As the load on the motor decreases, the phase angle tends to increase. In response the controller decreases the triac duty cycle which reduces the voltage applied to the motor and maintains the commanded phase angle. Conversely, as the motor load increases, the phase angle tends to decrease. In response the controller increases the triac duty cycle which increases the voltage applied to the motor and maintains the commanded phase angle.
Because of the analog nature of the Nola circuitry, that system is susceptible to changes during operation, due for example to variations in temperature. In addition, the Nola system requires a separate manual determination and setting of the power factor command potentiometer for each motor.